1. Field of the Invention
The present invention relates to analog filter circuits, and particularly to a digitally programmable active-RC filter.
2. Description of the Related Art
Active-RC (resistor-capacitor) filters based on operational amplifiers (op-amps) offer the highest dynamic range among the different filtering techniques. This is mainly due to their superior linearity compared with other techniques. However, the lack of efficient programmability features hinders their wide use in integrated circuits (ICs), wherein tuning properties are essential to compensate for process, component and temperature variations. The MOSFET-C approach was suggested to provide the missing tuning property, but it suffers from poor linearity, and hence limited dynamic range. Also, its tuning range is limited, particularly for low power supply voltages.
Digitally programmable devices are attractive for mixed digital-analog applications. The digital tuning property allows direct interfacing with the digital signal processing (DSP) part available in most modern systems, providing power-efficient design solution. Banks of resistors and/or capacitors can be employed in active-RC filters to offer the programmability feature. However, they occupy relatively large silicon area. In addition, the switching transistors (quasi-static switches) within the banks are associated with finite non-linear resistances.
Thus, a digitally programmable active-RC filter solving the aforementioned problems is desired.